I've just come across the site in the past few days, and i find it fascinating. It's dedicated to building a catalog of old photographs and linking them to the physical location where the photos where taken. The really neat part is that they try to match the photos with current images from Google Streetview.
I found the site because someone sent me a link to a demo video about the latest update. (I embedded the video above.) The value of this site as a research tool and a history lesson should be obvious. But if you're still curious, try one of the tours that have been made. Each tour is a series of photographs and their locations. It's really quite cool.